Method and composition useful treating metal surfaces

ABSTRACT

A composition suitable for treating metal surfaces prior to bonding of the surfaces to materials including metals, rubber, glass, polymers, sealants, coatings, and in particular polymeric adhesives, to enhance the strength of the bond and to prolong useful life in corrosive environments, is described. The composition comprises: (a) water; (b) metal alkoxide comprising M(OR) x , where M is a metal and R is an alkyl group; and (c) organoalkoxysilane comprising silane coupling functional groups capable of bonding with the material to be bonded to the metal surface; and (d) acid to promote hydrolysis and crosslinking of the metal alkoxide and organoalkoxysilane, wherein the molar ratio of metal alkoxide:organoalkoxysilane:acid is selected such that the composition is characterized by: (i) an extended shelf life, and (ii) is capable of crosslinking when applied to the metal surface to form a adherent coating having a substantially uniform distribution of metal, silicon, and oxygen species through the thickness of the coating, (iii) is capable of bonding with the material to be bonded to the metal surface to form a strong adherent bond between the metal surface and the material to be bonded to the metal surface. Also described is a method of using the composition to bond metal surfaces to one another.

BACKGROUND

This invention relates to a composition and method useful for treatingmetal surfaces to enhance bonding of the metal surfaces to othermaterials, and in particular to enhance bonding of metal surfaces usingpolymeric adhesives.

Materials such as metals, polymers, and ceramics are bonded to oneanother, or bonded to coatings such as polymers, enamel, glass, ceramic,magnetic ferrite, or refractory materials. Examples of industrialapplications include the bonding of structural metal or compositeassemblies using polymeric adhesives, widely used in the aircraftindustry, and increasingly used in the chemical engineering andautomobile industry. For example, aluminum alloy components are adhesivebonded to one another to form structural aircraft components havingreduced weights and which can be manufactured at lower costs. Otherexamples include powder painted aluminum components, and aluminum bondedto PTFE coatings to yield non-stick or low friction surfaces. In theseapplications the metal surfaces of the components are chemically treatedprior to bonding to promote adherence and corrosion resistance of theadhesive-bonded joint or interface between the metal components. Forexample, a typical three layer adhesive-bonded joint between twoaluminum alloy components comprises (1) an aluminum oxide layer on thealuminum component surface; (2) a primer layer on the oxide layer; and(3) an epoxy adhesive layer on the primer layer for bonding the aluminumcomponents to one another.

The durability and corrosion resistance of the joint between the metalsurface and the material bonded to the metal surface is particularlyimportant in structural applications, such as aircraft structures,because these joints are exposed to a wide range of environmentalconditions with extreme temperatures, high humidity, and highlycorrosive marine environments. To avoid failure of the joint as well asto meet stringent commercial passenger and cargo aircraft standards, theadhesive-bonded joint of the structural component must withstand theharsh environmental conditions, and in particular resistance tocorrosion and disbonding in humid salt laden environments, especiallythose resulting from sea spray or deicing materials. Failure of thesejoints often starts with diffusion of water through the adhesivefollowed by corrosion of the underlying metal structure. Thus it isdesirable to have a method and composition useful for bonding metalsurfaces that delays onset of corrosion and exhibits stability inaqueous and salt laden environments.

Conventional surface treatment processes have several disadvantages.Current pretreatment processes include anodizing the metal surface in abath of: (i) chromic acid as disclosed in U.S. Pat. No. 4,690,736;sulfuric acid as disclosed in U.S. Pat. No. 4,624,752; (ii) phosphoricacid; (iii) oxalic acid; or (iv) a mixture of sulfuric and chromicacids. These processes form a partially hydrated oxide coating on themetal surface. The partially hydrated oxide coatings corrode in humidenvironments, for example aluminum oxide coatings on aluminium surfacescorrode to form aluminum hydroxide, in particular a mixture of boehmite(Al₂ O₃ H₂ O) and pseudo-boehmite (Al₂ O₃ H₂ O), which is mechanicallyweak and adheres poorly to the aluminum metal. Further hydration leadsto formation of bayerite Al(OH)₃ which results in disbonding of thejoint. Thus, conventional anodizing processes can lead to hydrationinstability and failure of the joint.

Another disadvantage of chromate and phosphate based anodizing processesis that these processes typically use large amounts of water toneutralize the treated metal surfaces, and to rinse off the corrosiveacids used for anodization of the metal surface. Disposal of thephosphate or chromate containing waste water is expensive and can beenvironmentally hazardous. Commercial anodizing processes also requirelarge amounts of electricity to sustain an anodizing current in theanodizing bath, particularly for large metal components, and requireexpensive equipment such as large anodizing and rinsing tanks, automaticsystems for transferring the metal component from the anodizing tank tothe rinsing tank, and sizable electrical power supplies. Thus it isdesirable to have a bonding composition and method that does not useexcessive amounts of water or electricity, and that can be used withoutlarge capital outlays for expensive equipment.

Another disadvantage of conventional treatment processes is their narrowprocessing window. Deviation from the processing window can result inpoor bonding. For example, in phosphoric acid anodizing processes, ifthe metal component is not removed from the phosphoric acid bathimmediately after the anodization current is turned off, the anodizedoxide coating formed on the metal component can be rapidly dissolved bythe corrosive chromic or phosphoric acid bath, resulting in a looselybonded oxide coating. Thus, it is desirable to have a surface treatmentprocess that provides a relatively large processing window to allowflexible production schedules while minimizing failure of the bondedjoint.

Another significant disadvantage of conventional surface treatmentprocesses arises from their use of highly toxic and hazardous chemicals,such as hexavalent chromium compounds. Chromic compound rinses are usedto seal phosphoric acid treated metal surfaces to provide adequatecorrosion resistance. Disposal of the waste chromic byproducts, and thelarge amount of metal sludge dissolved in the acid, has becomeincreasingly expensive in view of stringent environmental regulationsand standards. Thus many conventional surface treatment processes arebeing gradually phased out because of the environmental regulations.Therefore, it is also desirable to have a non-toxic surface treatmentprocess that is substantially environmentally benign.

SUMMARY

The present invention satisfies these needs. One aspect of the inventionprovides a composition that is useful for treatment of metal surfacesprior to bonding of the metal surfaces to other materials, includingmetals, rubber, glass, polymers, sealants, coatings, and in particularpolymeric adhesives, to enhancing the strength of the bond, and toprolong useful life in corrosive environments. The compositioncomprises: (a) water; (b) metal alkoxide comprising M(OR)_(x), where Mis a metal and R is an alkyl group; (c) organoalkoxysilane comprisingsilane coupling functional groups capable of bonding with the materialto be bonded to the metal surface; and (d) acid to promote hydrolysisand crosslinking of the metal alkoxide and organoalkoxysilane. The molarratio of metal alkoxide:organoalkoxysilane:acid is selected such thatthe composition is characterized by: (i) an extended shelf life, and(ii) is capable of crosslinking when applied to the metal surface toform a adherent coating having a substantially uniform distribution ofmetal, silicon, and oxygen species through the thickness of the coating,(iii) is capable of bonding with the material to be bonded to the metalsurface to form a strong adherent bond between the metal surface and thematerial bonded thereto.

In another aspect, the invention provides a method for the treatment ofa metal surface prior to bonding of the metal surafec comprising thesteps of:

(a) forming a composition comprising (i) water, (ii) metal alkoxidecomprising M(OR)_(x), where M is a metal and R is an alkyl group; (iii)organoalkoxysilane comprising silane coupling functional groups capableof bonding with the material to be bonded to the metal surface, and (iv)acid to promote hydrolysis and crosslinking of the metal alkoxide andorganoalkoxysilane;

(b) coating the metal surface with the composition; and

(c) heating the composition to a temperature sufficiently high tohydrolyse and crosslink the composition to form an adherent coating onthe metal surface,

wherein the molar ratio of metal alkoxide:organoalkoxysilane:acid in thecomposition is selected such that the composition is characterized by:(i) an extended shelf life, and (ii) is capable of crosslinking whenapplied to the metal surface to form a adherent coating having asubstantially uniform distribution of metal, silicon, and oxygen speciesthrough the thickness of the coating, (iii) is capable of bonding withanother material to form a strong adherent bond between the metalsurface and the material bonded thereto.

In yet another aspect the invention provides a bonded metal structurecomprising an joint between at least two metal surfaces bonded to oneanother, the joint comprising:

(a) an adherent coating on each of the metal surfaces, each adherentcoating characterized by:

(i) a substantially uniform distribution of metal, silicon, and oxygenspecies through the thickness of the coating, and

(ii) the coating formed of a composition comprising: (1) water, (2)metal alkoxide comprising M(OR)_(x), where M is a metal and R is analkyl group; (3) organoalkoxysilane comprising silane couplingfunctional groups; and (4) acid to promote hydrolysis and crosslinkingof the metal alkoxide and organoalkoxysilane; and

(b) a polymeric adhesive between the adherent coatings,

wherein the molar ratio of metal alkoxide:organoalkoxysilane:acid in thecomposition is such that the adherent coating formed using thecomposition is capable of bonding with the metal surfaces and thepolymeric adhesive to form a strong and corrosion-resistant jointbetween the metal surfaces.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims and accompanying illustrative drawings,where:

FIG. 1 is a schematic of a scanning electron photo of a typical poresize distribution observed in a coating formed using the composition andmethod of the present invention; and

FIG. 2 is an auger electron spectrograph showing the uniformdistribution of metal, silicon and oxygen through the thickness of acoating formed using the composition and method of the presentinvention.

DESCRIPTION

The composition and method of the present invention is useful fortreatment of metal surfaces prior to bonding of the metal surfaces toother materials, including metals, rubber, glass, polymers, sealants,coatings, and in particular polymeric adhesives to enhance surfaceadhesion properties and prolong useful life in corrosive environments.The composition is particularly useful for promoting corrosionresistance and adhesion of metals, such as aluminum, steel, zinc,titanium, galvanized or plated metals, and alloys.

Although the surface treatment composition and process is illustrated byproviding an example of adhesive bonding of structural aluminum for airframes and automobiles (as used herein the word "aluminum" includes highpurity aluminum, commercial purity aluminum and aluminum based alloys,such as for example the 2000 series (Al-Cu alloys) and the 7000 series(Al-Zn-Mg alloys), the composition and method can also be used in otherapplications, for example to promote (i) paint adhesion, particularly ifcorrosion resistance is important as in air frames; (ii) polymeradhesion, for example PTFE bearings on aluminum, etc.; and (iii)adhesion of electroplated coatings. Thus, it should be appreciated thatmodifications made by those skilled in the art to the examples of thecomposition and method described herein, are within the scope of theinvention.

The composition of the present invention generally comprises (i) water;(ii) metal alkoxide comprising M(OR)_(x), where R is an alkyl group;(iii) organoalkoxysilane comprising silane coupling functional groupscapable of bonding with the material to be bonded to the metal surface;and (iv) acid to promote hydrolysis and crosslinking of the metalalkoxide and organoalkoxysilane. The molar ratio of metalalkoxide:organoalkoxysilane:acid is selected such that the compositionis characterized by: (i) an extended shelf life, and (ii) is capable ofcrosslinking when applied to the metal surface to form a adherentcoating having a substantially uniform distribution of metal, silicon,and oxygen species through the thickness of the coating, (iii) iscapable of bonding with the material to be bonded to the metal surfaceto form a strong adherent bond between the metal surface and thematerial bonded thereto.

The metal alkoxide and organoalkoxysilane are mixed together in thewater to form a stable liquid solution or dispersion useful for coatingthe metal surface. Preferably, the water comprises distilled anddeionized water. Optionally, the composition can also compriseconventional solvents useful for dissolving and dispersing organiccompounds. Both the water and solvent at least partially hydrolyse themetal alkoxide of the composition. Suitable solvents include any solventmiscible with water, and more preferrably miscible with the metalalkoxide and the organoalkoxysilane, including alchohols, such asmethanol, ethanol, and more preferably isopropanol.

The metal alkoxide of the composition typically has the stoichiometricformula M(OR)_(x), where R comprises at least one alkyl group having thestoichiometry C_(x) H_(2x+1), and M is a metal. Suitable metal alkoxidesinclude Si(OR)₄, Al(OR)₃, Ti(OR)₄, Zr(OR)₄, Ta(OR)₅, and Hf(OR)₄.Preferably, the metal component of the metal alkoxide is selected tomatch the metal of the metal surface to be coated to obtain optimalreactivity of the alkoxide with the metal surface. Matching the metal ofthe alkoxide to the metal surface allows the metal alkoxide to form astronger and more adherent coating on the metal surface which resistscorrosion and hydration in a humid environment. For example, Si(OR)₄ canbe used to bond silicon surfaces, Al(OR)₃ can be used to bond aluminumsurfaces, Ti(OR)₄ can be used to bond titanium surfaces, and Zr(OR)₄ canbe used to bond zirconium surfaces, Ta(OR)₅ can be used to bond tantalumsurfaces, and Hf(OR)₄ can be used to bond hafium surfaces.

The metal alkoxide is dispersed in the water using conventional methods.For example, one method of dispersing an metal alkoxide in watercomprises dripping the metal alkoxide into excess water under rigorousstirring. When aluminium alkoxide is dripped into water, the alkoxide ispartially hydrolysed to yield condensates that are crystalline above 80°C., and amphorous below 80°. While crystalline phase (boehmite) isunaffected by aging, the amorphous phase is unstable and can beconverted into crystalline boehmite when heated above 80° C. or intobayerite, Al(OH)₃, if kept below 80° C., in water. Both the crystallineand amorphous forms of aluminum monohydroxide (AlO(OH) are peptized toobtain clear alumina sol solutions by the acid present in the liquiddispersion. Typically, the composition comprises a colloidal dispersionhaving an average particle diameter from about 0.01 to about 1 micron.

The metal alkoxide can also be dissolved or suspended in a solvent thatis miscible with water. In the latter case, the metal alkoxide solutionis simply added to the water to form a liquid solution or dispersion,without extensive stirring or mixing.

The organoalkoxysilane compound used in the composition has silanefunctional components or groups that can react and bond to the materialto be bonded to the metal surface. Suitable organoalkoxysilane compoundsare selected from the group consisting of aminopropyltrimethoxysilane,aminopropyltriethoxysilane,N-beta-aminoethyl-gamma-propylmethyidimethoxysilane,N-beta-aminoethyl-gamma-propyltrimethoxysilane, divinyldimethoxysilane,divinyldi-beta-methoxyethoxysilane,di(gamma-glycidoxypropyl)dimethoxysilane, vinyltriethoxysilane,vinyltris-beta-methoxyethoxysilane, gamma-glycidoxypropyltrimethoxysilane, gamma-methacryloxypropyltrimethoxysilane andbeta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane. When a polymericadhesive is bonded to the metal surface, preferably, theorganoalkoxysilane contains an amine functionality. Preferredaminoalkoxysilanes include aminopropyltrimethoxysilane.

The acid of the composition is used to catalyze hydrolysis andcrosslinking of the metal alkoxide. Preferred acids include nitric acid,hydrochloric, acetic acid, dichloriacetic and monochloracetic andformiacid. Hydrofluoric, iodic, sulfuric, phosphoric boric, oxalic,phthalic, citric and carbamic acids can also be used. The molar ratio ofthe acid to water is preferably from about 0.0002 to about 20, and morepreferably from about 0.004 to about 17. The type and concentration ofthe acid catalyst can be tailored to yield coatings having linear,branched, two-dimensional, or three-dimensional crosslinked networks.

The molar ratio of metal alkoxide:organoalkoxysilane:acid is selectedsuch that the composition has an extended shelf life, preferably beingstable for at least about 1 week, and more preferably stable for atleast about 1 month. The stable shelf allows for fabrication, deliveryand storage of the premixed composition without undesirable hydrolysisor crosslinking of the composition.

The molar ratio of metal alkoxide:organoalkoxysilane:acid is selectedsuch that the composition is capable of forming an adherent coatinghaving pores extending through the at least a portion of the coating. Ithas been discovered that the diameter of the pores should be larger thanthe molecular size of the primer or adhesive material used to the bondthe metal surface to another material, which is typically from about 30to about 100 angstroms. Thus, preferably the diameter of at least about30%, and more preferably 50% of the pores, have an average pore diameterof at least about 0.01 micron, and more preferably an average porediameter of from about 0.01 to about 10 micron. Most preferably, atleast about 30% of the pores in the adherent coating extend throughsubstantially the entire thickness of the coating. A schematic of ascanning electron photo of a typical pore size distribution observed ina coating formed using the composition and method of the presentinvention is shown in FIG. 1. It is seen that a large number of pores,typically at least 30%, and more typically at least 50%, have an averagepore diameter of at least about 0.01 micron. The size, distribution andlength of the pores in the adherent coating allow for enhanced bondingbetween the metal surfaces and material to be bonded thereto, by forminga tortuous pore structure that allows the bonding material to physicallypermeate into, and couple with, the adherent coating, thereby forming astrong and corrosion resistant bond.

The molar ratio of metal alkoxide:organoalkoxysilane:acid is alsoselected such that the composition can form a adherent coating having asubstantially uniform distribution of metal, silicon, and oxygen speciesthrough the thickness of the coating, as shown in the auger electronspectrograph of FIG. 2. The uniform distribution of metal, silicon andoxygen through the thickness of a coating formed provides a relativelyhomogeneous bond between the metal surface and the material to be bondedto the metal surface. A homogeneous bond is advantageous when the bondis exposed to elevated or reduced operating temperatures, because thehomogeneous bond composition reduces the possibility of obtainingthermal expansion mismatch between non-homogeneous or anisotropic bondstructures. Reduced thermal expansion mismatch increases the usefulnessof the bonded structures formed using the composition of the presentinvention at sub-zero and hot temperatures.

A preferred molar ratio of metal alkoxide:organoalkoxysilane:acid whichprovides the desired characteristics for the composition is from about1:0.5:0.1 to about 1:15:0.8, and more preferably from about 1:1:0.2 toabout 1:8:0.5. In the preferred compositions the molar ratio of metalalkoxide:organoalkoxysilane is typically from about 0.01 to about 5, andmore typically from about 0.5 to 3.

When solvent is added to the composition, the molar ratio of metalalkoxide to organoalkoxysilane can change depending on the ratio ofwater to solvent. For example, when the solvent to water molar ratio isless than about 0.5, the molar ratio of metal alkoxide toorganoalkoxysilane is preferably from about 0.01 to about 5, morepreferably from 0.05 to 2, and most preferably from about 0.1 to about0.3. Thus, a suitable composition preferably comprises metal alkoxide ina concentration of about 0.3 to about 1.4 wt %, and more preferably from0.5 to 1.1 wt %; and organoalkoxysilane in a concentration of about 1 toabout 8 wt %, and more preferably from 1 to 6 wt %.

Similarly, when the ratio of solvent to water is greater than 2, themolar ratio of metal alkoxide to organoalkoxysilane is preferably fromabout 0.2 to about 1.2, more preferably from 0.4 to 0.9, and mostpreferably from about 0.6 to about 0.8. Thus, a suitable compositionpreferably comprises metal alkoxide in a concentration of about 0.5 toabout 4.5 wt %, and more preferably 1.5 to 3.5 wt %; andorganalkoxysilane in a concentration of about 0.5 to 4.5 wt %, and morepreferably from 1.7 to 3.7 wt %.

The composition described can also include additives such as rustpreventive compounds, for example molybdic acid or chromic acid; rustproofing agents such as phenolic carboxylic acids, for example tannicacid and gallic acid; and/or zirconium compound such as ammoniumzirconyl carbonate, to attain increased corrosion resistance.

A method for using the composition of the present invention to treatmetal surfaces prior to bonding of the metal surfaces will now bedescribed. It should be noted that the surface treatment according tothe invention is not only suitable in the manufacturing industry but isalso suitable for application by hand to small areas for repairingjoints. The metal surface is prepared prior to coating with thecomposition of the present invention, by degreasing and cleaning themetal surface using conventional methods to remove oil and greasecontaminants on the surface. Degreasing can be carried out usingtrichlorethylene vapor in a solvent vapor degreasing tank. The metalsurface is then scoured in an aqueous alkaline solution, such as a 10%sodium hydroxide solution, or degreased using conventional alkalinecleaners. Thereafter, the metal surface is rinsed in water by immersionin tap water for about 2-5 minutes.

Optionally, the cleaning treatment can be followed by a deoxidizingtreatment with water rinsing between the deoxidation steps. Suitabledeoxidizing solutions include solutions of TURCO-SMUT-GO #4,commercially available from Turco Products, Westminister, Calif.; or asolution comprising 27-36 wt % sulfuric acid and 22-35 g/L ferric iron.After immersing the metal surface in the deoxidizer solution for about10 to 12 minutes at 60° to 65° C., the surface is rinsed with warm tapwater for ten minutes and allowed to air dry.

The clean metal surface is dried and thereafter coated with thecomposition of the present invention, by dip coating, painting, sprayingor spin coating. In dip coating process, the substrate is dipped intothe coating composition and typically withdrawn vertically at a constantspeed. The thickness of the coating is primarily dependent on (i) theconcentration and viscosity of the coating composition, and (ii) thewithdrawal speed, because the film formed on the metal surface runs downalong the metal surface leaving behind a residue of coating composition.The substrate withdrawing speed can range from 0.5 cm/min to 40 cm/minand is preferably from 2.4 to 17.6 cm/min. During the dip coatingprocess, the coating composition can be maintained at any temperaturebelow the boiling point of the water and is preferably maintained atroom temperature. In the spray coating process, a thin uniform coatingcan be obtained by moving the substrate at a constant speed below anatomized spray of the coating composition.

After coating the metal surface with the coating composition, the coatedmetal surface is heat treated at low temperatures to remove residualsolvent, and to crosslink and polymerize the composition to form theadherent coating. The coated metal surface is typically heat to atemperature ranging from room temperature to 150° C., and more typicallyfrom 60° to 120° C., for about 30 to 60 minutes. The porosity and poresize of the coating can be controlled by the heat treatment and relativehumidity. The resultant adherent coating on the metal surface typicallyhas a thickness in the submicron range, and more typically from about0.05 to about 5 microns, and more typically from 0.1 to 1 micron.

It is believed that the water, metal alkoxide, organoaloxysilane andacid of the composition forms peptized alkoxide-silane colloidssuspended in the water. It is further believed that heat treatment ofthe coated composition on the metal surface, results in hydrolysis,crosslinking and condensation of the composition to provide an adherentcoating having a uniform distribution of metal, silicon and oxygen. In atypical hydrolysis reaction, an metal alkoxide ligand is replaced with ahydroxyl ligand as follows:

    M(OR).sub.z +H.sub.2 O - - - →M(OR)).sub.z-1 (OH+ROH)(1)

Thereafter, condensation reactions involving the hydroxyl ligandsproduce polymers having M--O--M or M--OH--M bonds, and by-product wateror alcohol. For example, condensation of an aluminum alkoxide comprisesthe sequential reactions:

    Al(OR).sub.2 OH+Al(OR).sub.3 - - -→(RO).sub.2 A--O(OR).sub.2 +ROH(2)

    Al(OR).sub.2 OH - - - →(RO).sub.2 Al--O--Al--O--Al(OR).sub.2 +H.sub.2 O                                                (3)

The resultant adherent coating contains functional groups that enhanceadhesion of the metal surface by coupling or polymerizing with thematerial to be bonded to the metal surface, and in particular with thepolymeric adhesive used to bond metal surfaces to one another. Anotheradvantage of the present composition is that tailored coatingmicrostructures with the desired pore sizes, volume, distribution andlengths can be formed in the adherent coating, to provide a tortuouspore structure that provides enhanced bonding capability.

Also, the adherent coating typically resists hydration by diffusedmoisture and protects the underlying metal surface from corrosion. Inaddition, the composition is environmentally benign and can be used tocoat large areas for mass production, at low costs, and without the useof toxic chemicals, and without use of large amounts of water orelectrical power. In this manner, the composition of the presentinvention provides an unusually adherent and corrosion resistantcoating.

An exemplary adhesive-bonded joint formed between two or more metalsurfaces using the composition of the present invention will now bedescribed. A typical adhesive-bonded joint between metal surfacescomprises of (i) adherent coatings of the composition of the presentinvention on the metal surfaces; (ii) optionally, a primer applied onthe coatings; and (iii) a polymeric adhesive, such as an epoxy, forbonding the primed metal surfaces to one another.

Firts the metal surfaces are coated with the composition of the presentinvention, and thereafter heat treated to hydrolyse and crosslink thecomposition. Thereafter, the coated surface is preferably primed using aprimer to reduce the chemical activity of the adherent coatings whichcan readily adsorb contaminants when exposed to the atmosphere, yieldinginferior joints. Any conventional primer can be used to prime the coatedmetal surface, the primer being selected according to the polymericadhesive used in the bonding process. Primers containing a ketone-typesolvent and chromium as a corrosion inhibitor are suitable but areundesirable because of their environmental toxicity. Water-basedadhesive primers such as the 6747 and 6757 primer, commerciallyavailable from Cytech Company, Weston, Mich., are preferred because oftheir reduced environmental toxicity, and typically require a 120° C.cure.

A polymeric adhesive, such as an epoxy adhesive is applied to the coatedmetal surfaces for bonding the metal surfaces to one another. Commonepoxy adhesives include the EA-9649 resin, commercially available fromDexter Company, or the FM73, FM-300 or FM-330 resins commerciallyavailable from the aforementioned Cytech Company.

After application of the polymeric adhesive to the coated metalsurfaces, the metal surfaces are joined to one another, and the joint isfirmly held during heat treatment at a temperature and pressure suitablefor curing and bonding the metal surfaces to one another. For examplefor the FM-73 film adhesive from Cytech Company, a suitable heattreatment is at a temperature of from about 120°C. for about 60 minutes,under an applied pressure of about 40 psi.

Metal surfaces joined using the composition and process of the presentinvention have high shear strengths and good corrosion resistance inharsh environments. The shear strength of a metal joint fabricated usinga water based composition of the present invention as measured by thelap shear joint test described in ASTM D 100-72, was typically above5500 psi, and often close to 5900 psi. These shear strengths areequivalent to the shear strengths obtained using conventional phosphoricacid anodizing processes. Also, the coatings exhibited goodthermodynamic and hydrolytic stability as well as corrosion resistance.Further, coated aluminum alloy samples showed no difference inmechanical fatigue tests compared to untreated samples.

The environmental stability of the joints formed using the process ofthe present invention were tested using a wedge crack test according toASTM 3762-79. Within the tested range, the coatings yieldedsubstantially equivalent crack growth than that obtained from phosphoricanodizing processes., which indicates good environmental stability.Typical initial crack lengths from the wedge crack test were in therange of 1.1 inch to 1.3 inch.

EXAMPLE 1

The following examples demonstrate the suitable of the composition andmethod for coating and bonding aluminium alloy substrates. Aluminumalloy 2024-T3 substrates to be coated by the method of the presentinvention were cleaned by immersion in a warm degreasing Brulin 815Ddetergent (diluted 1:15 in water) for 10 minutes until surface dirt wasremoved, followed by a warm tap water rinse for 10 minutes. Aftercleaning, the substrates were immersed in warm alkaline ISOPREP 44cleaning solution (60g/L in water) for five minutes and then rinsed withwarm tap water for 10 minutes. The cleaned substrates were then immersedin a deoxidizer solution before applying coatings. The deoxidizersolution was prepared by dissolving 13 g of TURCO-SMUT-GO #4 in oneliter of 14% nitric acid solution. After immersed in the deoxidizersolution for 10 minutes, the aluminum alloys were rinsed with warm tapwater for ten minutes and allowed to air dry.

The coating composition was made by adding 140 ml (0.55 mole) ofaluminum sec-butoxide to 900 ml of distilled water, while stirring at80° C. for 15 minutes. The resultant solution was allowed to cool downbefore adding 10 ml of 70% nitric acid (0.15 mole). The solution wasthen refluxed at 80° C. for a few hours until a slightly cloudy solutionwas obtained. This water-based aluminum alkoxide solution was thendiluted to 1/16 its original concentration. 4 ml of3-aminopropyltrimethoxysilane was slowly added to 100 ml of the dilutedsolution while stirring to yield a coating composition with a colloidaldispersion.

The dry substrates were coated in a bath of the coating compositionusing the dip-coating method. The substrate withdrawing speed variedfrom 17.6 cm/min to 2.4 cm/min. The coated substrates were dried in airand then heat treated in an oven at 125° C. for one hour. Thereafter,BR127 primer manufactured by Cytech Company was applied on the coatedsurface of the substrates using a paintbrush. The primer was cured at120° C. for 30 minutes in an oven. The substrates were then bonded usingFM73 film adhesive manufactured by Cytech Company and the bondedsubstrates were cured in a platen press for one hour at 120° C. with 40psi pressure. The shear strength of the lap shear specimens weredetermined on an Applied Test Systems model 900 using a cross-head speedof 0.05 in/min. The lap shear strength of the bonded metal surfaces wereequivalent to those of joints assembled from samples anodized by aphosphoric acid anodization process, demonstrating the equivalent bondstrength of the present coatings. The failure is at or within theadhesive, and not in the bonding layer, indicating that the bond formedby the coating is stronger than the adhesive.

The wedge crack specimens were cracked according to ASTM D-3762-79, theinitial crack length was marked, and the specimen then were placed in ahumidity chamber maintained at 95% relative humidity and 60° C. Crackgrowth was measured using a microscope at 15 times enlargement after onehour in the chamber. The wedge crack test data shows that jointsassembled from substrates which were treated with the coatingcomposition of the present invention exhibited small crack growths inthe range of 0.03 to 0.05 inch. Minimal crack growth is an indication ofgood hydrothermal stability of the bonded joint. Failure also occurredin the adhesive layer, not in the bonding layer, indicating that thebond formed by the coating is stronger than the adhesive.

EXAMPLE 2

This example is similar to example 1 except that a different coatingcomposition was used. The coating composition used in this example wasprepared by adding 18 ml of 3-aminopropyltrimethoxysilane and 18 ml ofaluminum sec-butoxide to 180 ml of dry isopropanol. The solution waswell mixed, and 1 ml of 99.5% acetic acid added after mixing. Theresultant solution was stirred overnight at room temperature in a closedbottle to reduce exposure to air or moisture, to form the coatingcomposition. The coating composition was then coated on aluminium alloysubstrates, which were then crosslinked and bonded as described above.

The lap shear strength of substrates bonded using this coatingcomposition were equivalent to joints bonded from samples anodized withphosphoric acid. Furthermore, the wedge crack test show that the jointsexhibited small crack growth in the range of 0.01 to 0.05 inch.

EXAMPLE 3

This example is similar to example 1 except that a nonchromateddeoxidizer was used to deoxidize the substrate, the deoxidizing solutionbeing a 27.5% nitric acid solution. The substrates to be coated weredeoxidized in the nitric acid solution for one hour followed by warmwater rinse. Thereafter, the coating composition and method described inExample 1 was used to bond the aluminium alloy substrates to oneanother. The lap shear strength of joints of substrates prepared usingthe nonchromated deoxidizing process had average strengths of 5983 psi,and the wedge crack tests yielded crack growth of 0.066 inch.

EXAMPLE 4

This example is similar to example 1 except that instead of using aprimer before bonding, a 10% solution of 3-aminopropltrimethoxysilane inisopropanol was used as a primer. The silane solution was applied ontothe coated surface using a paintbrush and then cured at 120° for 30minutes. Thereafter, the coating composition and method described inExample 1 was used to bond the aluminium alloy substrates to oneanother. The lap shear strength of joints of bonded substrates preparedusing the non-chromated, non-commercial primer were an average of about5379 psi.

The composition and process of the present invention has numerousadvantages. The composition is non-toxic and allows use of anenvironmentally benign surface treatment process for metal surfaces thatenhances bonding strength, delays onset of corrosion, and exhibitsstability in aqueous and salt laden environmental conditions. Thesurface treatment process also provides a relatively large processingwindow, and minimizes failure of adhesive bonded joints. Further,excessive amounts of water or electricity are not required. Also, thesurface treatment process can be applied by painting or spraying anddoes not require large capital outlays.

Although the present invention has been discussed in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. For example, the coating composition, treatment time, andcoating process conditions be varied from those specifically disclosedto produce a surface coating tailored for a particular surfacecomposition to provide a mechanically strong and corrosion resistantbond. Other additives known in the art may also be included in thetreatment process to further improve bonding of the metal surface,including for example, acids such as oxalic, chromic and malonic acids.Therefore, the spirit and scope of the appended claims should not belimited to the description of the preferred versions contained herein.

What is claimed is:
 1. A composition for treatment of a metal surfaceprior to bonding of the metal surface to another material, thecomposition consisting of:(a) water; (b) metal alkoxide of the formulaM(OR)_(x), where R is an alkyl group; (c) a single organoalkoxysilanecomprising silane coupling functional groups for bonding with thematerial to be bonded to the metal surface, the organoalkoxysilane beingselected from the group consisting of aminopropyltrimethoxysilane,aminopropyltriethoxysilane, N-beta-aminoethyl-gamma-propylmethyldimethoxysilane,N-beta-aminoethyl-gamma-propyltrimethoxysilane, divinyldimethoxysilane,divinyldi-beta-methoxyethoxysilane,di(gamma-glycidoxypropyl)dimethoxysilane, vinyltriethoxvsilane,vinyltris-beta-methoxyethoxysilane, gamma-glycidoxypropyltrimethoxysilane, gamma-methacryloxypropyl-trimethoxysilane andbeta-(3.4-epoxycyclohexyl)ethyltrimethoxysilane; (d) acid to promotehydrolysis and crosslinking of the metal alkoxide andorganoalkoxysilane; and (e) optionally, a solvent capable of dissolvingthe metal alkoxide, wherein the molar ratio of metalalkoxide:organoalkoxysilane:acid is about 1:0.5:0.1 to about 1:15:0.8,and is selected such that the composition provides a liquid dispersionhaving an extended shelf life of at least about 1 week.
 2. Thecomposition of claim 1, wherein the molar ratio of metalalkoxide:organoalkoxysilane:acid is from about 1:1:0.2 to about 1:8:0.5.3. The composition of claim 1, wherein the molar ratio of metalalkoxide:organoalkoxysilane is from about 0.01 to about
 5. 4. Thecomposition of claim 3, wherein the molar ratio of metalalkoxide:organoalkoxysilane is from about 0.5 to
 3. 5. The compositionof claim 1, wherein the metal M of the metal alkoxide is the same metalas the metal surface.
 6. The composition of claim 1, wherein the metalalkoxide is selected from the group consisting of Si(OR)₄, Al(OR)₃,Ti(OR)₄, Zr(OR)₄, Ta(OR)₅, and Hf(OR)₄.
 7. The composition of claim 1,wherein the organoalkoxysilane is aminoalkoxysilane.
 8. The compositionof claim 7, wherein the organoalkoxysilane isaminopropyltrimethoxysilane.